Optics arrangements including light source arrangements for an active matrix liquid crystal generator

ABSTRACT

A system for producing modulated lights disclosed. The system comprises a spatial light modulator including a light modulating medium switchable between different states so as to act on light in ways which form overall patterns of modulated light. The system also includes an arrangement for switching the modulating medium between the different states in a controlled way and an illumination arrangement for producing a source of light. The system, further includes an optics arrangement for directing light from the source of light into the spatial light modulator and for directing light from the spatial light modulator through a predetermined source imaging area. The optics arrangement cooperates with the illumination arrangement and the spatial light modulator so as to produce a real image of the source of light within the source imaging area such that an individual is able to view a virtual image of the overall patterns of modulated light from the source imaging area. A variety of novel optics arrangements are disclosed including specific combinations of different light sources, diffusing plates, polarizers, beam splitters, analyzers, lenses, mirrors, and holographic optical elements which allow the overall optical arrangement to be miniaturized to the same degree and in coordination with the spatial light modulator. The different light sources include using a plurality of light sources, such as LEDs, to form an array of light sources, each of the light sources providing light to a corresponding portion of the spatial light modulator.

GOVERNMENT CONTRACT CLAUSE

This invention was made with Government support under contractsNAS9-18858 and NAS9-19102 awarded by the National Aeronautics and SpaceAdministration and contracts DAA-H01-92-C-R275 and DAA-H01-94-C-R154awarded by the Advanced Research Projects Agency. The Government hascertain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to image generating systems, andmore particularly to optics arrangements and light source arrangementsespecially suitable for miniaturized image generating systems such asthe miniaturized image generator disclosed in U.S. Pat. No. 5,748,164entitled ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, which isincorporated herein by reference.

One of the ongoing challenges facing the manufacture of miniature imagegenerating systems is providing smaller and smaller systems. Miniatureimage generating systems which are small enough to be mounted onto ahelmet or small enough to be supported by a pair of eyeglasses will finda wide variety of uses if they can provide adequate resolution andbrightness in a small, low-power package at a low cost. Conventionaltechnologies such as CRTs are difficult to miniaturize and therefore donot hold much promise in this field. Alternatively, new systems based onVLSI integrated circuits are currently being developed which providemuch smaller spatial light modulators for use in a miniaturized imagegenerating systems. However, one of the problems in this field isproviding optics and illuminating arrangements which may be scaled downin coordination with the miniaturized spatial light modulator in orderto provide an overall image generating system which is practical andcompact enough to be mounted onto a helmet or supported by a pair ofglasses. Another problem in this field is providing an illuminatingarrangement which requires as little power as possible in order to makethe overall system more portable.

Referring to FIG. 1, a prior art miniature image generator systemgenerally designated by reference numeral 10 will be described. System10 includes a transmissive spatial light modulator 12 which modulateslight from a light source 14 positioned immediately adjacent to spatiallight modulator 12 by selectively changing the polarization of lightpassing through the spatial light modulator. A polarizer 16 ispositioned between light source 14 and spatial light modulator 12 whichallows light of one polarization from light source 14 to enter spatiallight modulator 12. An analyzer 18 is positioned adjacent to theopposite side of spatial light 12 which allows light of a particularpolarization to pass through analyzer 18. An eyepiece lens 20 having afocal length F1 is positioned approximately one focal length F1 fromspatial light modulator 12 such that a viewer may see a virtual image ofthe pattern of modulated light formed by spatial light modulator 12 whenthe viewer's eye is positioned in an appropriate location. As shown inFIG. 1, this arrangement results in a viewing region indicated byreference numeral 22 from which a viewer may view the entire virtualimage of the pattern of modulated light produced by the spatial lightmodulator display.

In the above described arrangement, since light source 14 is positionedadjacent to spatial light modulator 12, light source 14 must have alight emitting surface with essentially the same surface area as spatiallight modulator 12. Also, in order for the optics to perform properly,the light source is a diffuse light source. However, these requirementscauses two major problems. First, a large diffuse light source asdescribed above is substantially more expensive than other types oflight sources. Second, because light source 14 is diffuse, a largepercentage of the light generated by light source 14, indicated by lines24, is directed to areas which are not within viewing region 22including areas in which the light does not pass through eyepiece lens20. This wastes a large percentage of the light produced by light source14 and requires much more light to be produced than would be necessaryif substantially all of the available light were directed into viewingregion 22. This wastage of light significantly increases the powerrequirements of the overall system. As will be seen hereinafter, thepresent invention provides a variety of novel optics arrangementsincluding novel light source arrangements which, when combined withminiaturized spatial light modulators, are capable of providing lowpower, compact miniaturized image generating system is that may be usedto produce a direct view miniature display.

SUMMARY OF THE INVENTION

As will be described in more detail hereinafter, a system for producingmodulated light is disclosed. The system comprises a spatial lightmodulator including a light modulating medium switchable betweendifferent states so as to act on light in ways which form overallpatterns of modulated light. The system also includes means forswitching the modulating medium between the different states in acontrolled way and illumination means for producing a source of light.The system further includes optics means for directing light from thesource of light into the spatial light modulator and for directing lightfrom the spatial light modulator through a predetermined source imagingarea. The optics means cooperates with the illumination means and thespatial light modulator so as to produce a real image of the source oflight within the source imagine area such that an individual is able toview a virtual image of the overall patterns of modulated light from thesource imaging area.

In one preferred embodiment of the present invention the spatial lightmodulator is a reflective type spatial light modulator and the opticsmeans cooperate with said illumination means and said spatial lightmodulator such that some of the light passing from the illuminationmeans to the spatial light modulator overlaps with some of the lightpassing from the spatial light modulator to the source imaging area.

In another embodiment of the present invention, the light source isprovided by means of an array of light emitting sources such as LEDs(light emitting diodes) spaced apart by a predetermined distance. Thesespaced apart light sources, in combination with the optical components,produce an equal plurality of images at the source imaging area whichare spaced apart from one another by a predetermined distance. Theoptical components of this embodiment may include a single collimatinglens disposed optically between the light sources and the spatial lightmodulator, or alternatively, may include a plurality of collimatinglenses, each of which is disposed optically between an associated one ofthe light sources and the spatial light modulator so as to direct lightfrom its associated light source to a corresponding portion of thespatial light modulator.

In the case of a plurality of collimating lenses, the optical componentsalso include a single eyepiece lens which is disposed optically betweenthe spatial light modulator and the source imaging area and whichdefines a much greater focal length than the focal length of each of theindividual collimating lenses. Also, the light sources may be disposedoptically approximately a focal length away from their associatedcollimating lens, such that the plurality of images produced at thesource imaging area are substantially larger than their respective lightsources. Alternatively, in this arrangement, the light sources aredisposed optically slightly closer to their associated collimating lensthan one focal length so as to cause each collimating lens to directlight from its associated light source to the spatial light modulator ina slightly diverging manner. The spatial relationship between the lightsources and the divergence of the light from the collimating lenses aresuch that the plurality of images produced at the source imaging areaoverlap one an other in a predetermined way.

The plurality of light sources may be provided in a variety ofarrangements, in a first arrangement, the arrangement includes a singledielectric substrate having on one surface a pattern of electricallyconductive leads adapted for connection to a source of electric power. Aplurality of LEDs are individually attached to the substrate andelectrically connected with the pattern of leads. An equal plurality ofindividual collimating lenses are attached to the substrate and disposedoptically over associated ones of the LEDs. In a second arrangement, thearrangement includes a single LED wafer having on one surface a patternof electrically conductive leads adapted for connection to a source ofelectric power. The pattern of leads divides the wafer into theplurality of LEDs. An equal plurality of individual collimating lensesmay be attached to the wafer and disposed optically over associated onesof the LEDs. Alternatively the arrangement includes a single substratewhich is attached to the LED wafer and which is integrally formed todefine an associated collimating lens for each of the LEDs. In a thirdarrangement which may be any combination of the first and secondarrangement, the plurality of LEDs include LEDs of different colorsthereby providing a color version of the miniaturized assembly.

In a color version of the present invention, the light sources includedifferent color light sources, such as LEDs, which are spaced apart apredetermined distance d and which emit light outwardly at a maximumangle A. A light diffusing plate is spaced from the light sources adistance L. Thus, the positional relationship between the light sourcesand the diffusing plate is such that L is at least approximately equalto d/A. In this way, as will be seen, it is possible to obtain properregistration of the different color images even though the light sourcesare spaced apart from one another.

As will be described in more detail hereinafter, a variety of specificarrangements for the optical components of the system for producingmodulated light are also disclosed. These arrangements include specificcombinations of a variety of light sources, polarizers, beam splitters,analyzers, lenses, mirrors, and holographic optical elements arranged todirect the light from the light source into the spatial light modulatorand from the spatial light modulator to the source imaging area.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings in which:

FIG. 1 is a diagrammatic side view of a prior art miniaturized imagegenerating system;

FIG. 2A is a diagrammatic side view of a miniature image generatingsystem designed in accordance with the present invention having a lightsource positioned away from the spatial light modulator and includingoptical elements which form a real image of the light source at a sourceimaging area and allow a viewer to view a virtual image of a pattern ofmodulated light formed by a spatial light modulator when the pupil ofthe viewer's eye is positioned in the source imaging area;

FIG. 2B is a diagrammatic side view of a basic reflective typeminiaturized image generating system designed in accordance with thepresent invention which illustrates all of the elements of a particularoptical system for the miniaturized image generator including a lightsource, a spatial light modulator, an eyepiece a source imaging area,and a polarizing beam splitting cube for directing one polarization oflight from the light source into the spatial light modulator and fordirecting the opposite polarization of light from the spatial lightmodulator to the eyepiece which directs the lip-t to the source imagingarea forming a real image of the light source within the source imagingarea;

FIG. 3 is a diagrammatic side view of one embodiment of a miniaturizedimage generating system designed in accordance with the presentinvention including a plurality of light sources which, in combinationwith the other optics components, produce a corresponding real image ofthe plurality of light sources at the source imaging area;

FIG. 4 is a diagrammatic side view of second embodiment of aminiaturized image generating system designed in accordance with thepresent invention including a plurality of light sources and a pluralityof collimating lenses each of which is associated with a correspondinglight source, which, in combination with the other optics components,produce a corresponding real image of the plurality of light sources atthe source imaging area;

FIGS. 5A and 5B are diagrammatic side views illustrating the opticalrelationship between the collimating lenses and the eyepiece lenses ofFIG. 2 and FIG. 4;

FIG. 6 is a diagrammatic side view of the image generator of FIG. 4 inwhich the light sources are positioned slightly closer to theirassociated collimating lens than one focal length so as to cause eachcollimating lens to direct light from its associated light source to thespatial light modulator in a slightly diverging manner;

FIGS. 7A and 7B are diagrammatic perspective views of light sourcearrangements designed in accordance with the present invention for usein, for instance, the miniature image generator of FIG. 4;

FIG. 8 is a diagrammatic side view of a third embodiment of aminiaturized image generating system designed in accordance with thepresent invention including an auxiliary polarizer positioned opticallybetween the light source and the spatial light modulator;

FIG. 9 is a diagrammatic side view of the miniaturized image generatingsystem of FIG. 8 including an auxiliary analyzer positioned opticallybetween the spatial light modulator and the source imaging area;

FIG. 10 is a diagrammatic side view of a fourth embodiment of aminiaturized image generating system designed in accordance with thepresent invention including an polarizer positioned optically betweenthe light source and the spatial light modulator, an analyzer positionedbetween the spatial light modulator and the source imaging area, and acurved surface arrangement for directing the light from the light sourceto the spatial light modulator and transmitting the light from thespatial light modulator to the eyepiece which directs the light to thesource imaging area;

FIG. 11 is a diagrammatic side view of the miniaturized image generatingsystem illustrated in FIG. 10 in which the polarizer and analyzer areformed as part of the curved surface arrangement;

FIG. 12 is a diagrammatic side view of a fifth embodiment of aminiaturized image generating system designed in accordance with thepresent invention including a holographic polarizing beam splitterpositioned optically between the light source and the spatial lightmodulator and between the spatial light modulator and the source imagingarea;

FIG. 13 is a diagrammatic side view of a sixth embodiment of aminiaturized image generating system designed in accordance with thepresent invention including an edge-illuminated holographic illuminator;

FIG. 14A and 14B are diagrammatic side views of a seventh embodiment ofa miniaturized image generating system designed in accordance with thepresent invention in which the spatial light modulator is directlyilluminated by the light source without other optics components fordirecting the light into the spatial light modulator;

FIG. 15A and 15B are diagrammatic side views of an eighth embodiment ofa miniaturized image generating system designed in accordance with thepresent invention in which the spatial light modulator is directlyilluminated by the light source without other optics components fordirecting the light Into the spatial light modulator and the lightsource is positioned between the spatial light modulator and theeyepiece lens;

FIG. 16 is a diagrammatic side view of a ninth embodiment of aminiaturized image generating system designed in accordance with thepresent invention including an arrangement for converting light which isnot directed into the spatial light modulator by the polarizing beamsplitting cube to the opposite polarization and redirecting it back intothe polarizing beam splitting cube;

FIG. 17 is a diagrammatic side view of a tenth embodiment of aminiaturized image generating system designed in accordance with thepresent invention including a arrangement for converting light which isnot directed into a first portion of the spatial light modulator by afirst polarizing beam splitting cube to the opposite polarization anddirecting it into a second polarizing beam splitting cube associatedwith a second portion of the spatial light modulator;

FIG. 18A-C are diagrammatic views of an eleventh embodiment of aminiaturized image generating system designed in accordance with thepresent invention; and

FIG. 19 is a diagrammatic side view of a portion of a miniaturized imagegenerating system illustrating a plurality of light sources of threedifferent colors, a collimating lens, and a polarizing beam splittingcube tuned to a first one of the three different colors of light, and inwhich the light sources of the other two colors are positioned tocooperate with the collimating lens to direct their light to thepolarizing beam splitting cube at angles which improve the efficiency atwhich the polarizing beam splitting cube acts upon the light of the twoother colors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIGS. 2-18, wherein like components are designated by likereference numerals throughout the various Figures, attention isinitially directed to FIG. 2A. This Figure illustrates the generaloptical elements of an optical system, designed in accordance with thepresent invention, for an image generating system, or miniaturizedassembly for producing modulated light, including a spatial lightmodulator. In this case, the system is a miniature display systemgenerally indicate, by reference numeral 26. As shown in FIG. 2A, asuitable and readily providable light source 28 is positioned away froma transmissive spatial light modulator 30 having an writing arrangement32 for controlling the light modulating states of spatial lightmodulator 30. Writing arrangement 32 may also switchably control lightsource 28. Spatial light modulator 30 modulates light from light source28 by selectively changing the polarization of the light passing throughthe spatial light modulator in response to data signal from writingarrangement 32. A collimating lens 34 is positioned between light source28 and spatial light modulator 30 and an eye piece lens 36 is positionedbetween spatial light modulator 30 and a source imaging area 38 suchthat substantially all of the light generated by light source 28 isdirected through source imaging area 38 except for any light which isspecifically absorbed by or directed away from source imaging area 38 byother optical elements positioned within the optical path between lightsource 28 and source imagining area 38 such as, for example a polarizer40 or an analyzer 41. Eyepiece lens 36 having a focal length F2 ispositioned one focal length F2 from spatial light modulator 30 andcooperates with light source 28, collimating lens 34, and spatial lightmodulator 30 to form a real image of light source 28 at source imagingarea 38 such that a virtual image of the pattern of modulated light fromspatial light modulator 30 is directly visible by a viewer from aviewing region 42. The real image of light source 28 is formed at sourceimaging area 38 because light source 28 is positioned a distance morethan F2, the focal length of eyepiece lens 36, from eyepiece lens 36.

The above described arrangement illustrated in FIG. 2A has the advantageover the prior art of directing a much greater percentage of the lightfrom light source 28 through source imaging area 38 and into viewingregion 42. This significantly reduces the power requirement for thelight source since the wastage of light described above for the priorart arrangement is significantly reduced if not eliminated. Also, asystem designed in accordance with the present invention allows a widevariety of light sources to be used including light sources which aresubstantially less expensive than the large diffuse light source 14 usedin the prior art system. However, this particular arrangement shown inFIG. 2A substantially increases the overall length of the system andtherefore is not practical when miniaturization of the overall system isimportant.

Referring now to FIG. 2B, an alternative basic configuration of anoverall display system designed in accordance with the present inventionand generally designated by reference numeral 44 will be described.Display system 44 includes light source 28, collimating lens 34,eyepiece lens 36, and source imaging area 38 as describe above for FIG.2A. However, in this embodiment of the present invention, a reflectivetype spatial light modulator 46 controlled by writing arrangement 32 isused instead of a transmissive spatial light modulator. As shown in FIG.2B, a suitable and readily providable polarizing beam splitting cube 48is positioned between spatial light modulator 46 and eyepiece lens 36.Also, light source 28 and collimating lens 34 are positioned to one sideof polarizing beam splitting cube 48.

During the operation of basic display system 44 described above, lightfrom light source 28, indicated by lines 49, is collected by collimatinglens 34 and directed into polarizing beam splitting cube 48. Thepolarizing beam splitting cube reflects light of one polarization, forexample S-polarized light, into spatial light modulator 46 and wasteslight of the opposite polarization, for example P-polarized light,allowing it to pass through polarizing beam splitting cube 48. Spatiallight modulator 46, controlled by writing arrangement 32, acts on thelight of the one polarization (S-polarized light) directed into themodulator by converting certain portions of the light of the onepolarization (S-polarized light) to light of the opposite polarization(P-polarized light) forming an overall pattern of modulated light thatis reflected back into polarizing beam splitting cube 48. The polarizingbeam splitting cube wastes light of the one polarization (S-polarizedlight) by reflecting it back toward light source 28 and allows theconverted light of the opposite polarization (P-polarized light) to passthrough polarizing beam splitting cube 48 into eyepiece lens 36 forminga real image of light source 28 at source imaging area 38. As describedabove, the real image of light source 28 is formed at source imagingarea 38 because light source 28 is positioned optically a distancegreater than one focal length of eyepiece lens 36 from eyepiece lens 36.This arrangement also produces a virtual image of the pattern ofmodulated light that is viewable from the source imaging area andviewing region 42. One specific novel arrangement for spatial lightmodulator 46 and writing arrangement 32 is disclosed in U.S. Pat. No.5,748,164.

As illustrated by FIG. 2B, the above described arrangement, whichincludes a reflective type spatial light modulator such as spatial lightmodulator 46, allows light source 28 to be moved away from spatial lightmodulator 46 without increasing the front to back length of the overallsystem as was shown in FIG. 2A. This system, designed in accordance withthe present invention, folds the optical path such that the portion ofthe optical path in which light from the light source is directed intothe spatial light modulator overlaps the portion of the optical path inwhich the light is directed from the spatial light modulator to theeyepiece lens. By overlapping the optical path as described, the samephysical space is used for both of these purposes and therefore thelength of the system is not increased relative to the prior art systemdescribed above and shown in FIG. 1. In the embodiment illustrated inFIG. 2B, this folding of the optical path is accomplished by positioningpolarizing beam splitting cube 48 in the space between spatial lightmodulator 46 and eyepiece 36. Again, this does not increase the lengthof the system because, as shown in FIG. 1 and 2A, the eyepiece lens mustbe positioned approximately one focal length of the eyepiece lens awayfrom the spatial light modulator which provides sufficient space for thepolarizing beam splitting cube.

By moving light source 28 away from the spatial light modulator asspecified by the present invention in order to form a real image oflight source 28 at source imaging area 38, optical elements may be addedto the system which direct the light from source 28 into spatial lightmodulator 46 in a controlled way. A variety of optical elements, whichwill be described in more detail hereinafter, may be used to direct thelight from source 28 into the spatial light modulator and from thespatial light modulator so as to form a real image of light source 28 atsource imaging area 38. As described above, these optical elements mayalso be arranged to allow a virtual image of the overall pattern ofmodulated light produced by the spatial light modulator, in other wordsa virtual image of the display, to be visible from source imaging area38 and viewing region 42. Also as mentioned above, this arrangement ofthe present invention provides the substantial benefit of being able todirect a much larger percentage of the light generated by light source28 into source imaging area 38 when compared with prior art systems.This avoids wasting light by directing light into regions other thanviewing region 42, or in other words, regions from which a viewerviewing the display would not be able to view the entire virtual imageof the pattern of modulated light produced by the spatial lightmodulator. Therefore, a system designed in accordance with the presentinvention more efficiently uses the light produced by the light sourcewhen compared with prior art image generating systems which reduces thepower requirements of the overall system. Furthermore, a wide variety ofdifferent light sources may be used including less expensive lightsources than prior art systems require.

Although the basic optical elements of the display system illustrated inFIG. 2B are functional, as the overall system is scaled down in size, itbecomes more and more difficult to scale down the optical elements tothe same degree. Also, even though the optical paths upstream anddownstreams of the spatial light modulator overlap, the arrangementshown in FIG. 2B adds to the bulk of the system because light source 28is positioned somewhat off to the side of the rest of the system.Furthermore, since a light source with a very small spatial extent isbeing used, the “exit pupil”, that is the size of the real image of thesource at the source imaging, area, becomes so small that normalmovement of a viewer's eye and tolerances for exact positioning of theviewer's eye result in the viewer's eye, at times, being moved such thatall or portions of the virtual image of the display are not viewable.Also, as the system is scaled down in size, the eye relief, that is thedistance from the eyepiece lens to the viewer's eye, indicated bydistance R in FIG. 2B, is reduced. In the case of a helmet mounteddisplay, the desired eye relief is, for example, approximately 25 mmwhich allows enough space for a viewer wearing eyeglasses to comfortablyuse the display. At distances less than 25 mm this may become a problemwhere eye glasses are concerned. Both of these viewing characteristics,that is exit pupil and eye relief, are important to the functionality,of the system, and, along with the overall bulk of the opticalcomponents used, are major considerations when reducing the size of aminiaturized image generating system. As will be described in moredetail hereinafter, the present invention provides a variety of novelarrangements which address these and other problems.

Referring now to FIG. 3 which illustrates a miniaturized display systemgenerally indicated by reference numeral 50, a first particularembodiment of the present invention will be described in detail. Asshown in FIG. 3, miniaturized display 50 includes spatial lightmodulator 46, collimating lens 34, polarizing beam splitting cube 48,and eyepiece lens 36 as were described above for FIG. 2B. However, inaccordance with the present invention, display 50 includes an array ofor a plurality of individual light sources which are indicated byreference numeral 52. In this particular embodiment and in accordancewith one aspect of the present invention, the array of light sourcesincludes LEDs, specifically three rows of three LEDs. Light sources 52are spaced apart so as to, in cooperation with the optics components,produce a real image of an equal array or plurality of the sources atsource imaging areas 54. Although only three rows of three light sourcesare described, it should be understood that the array of light sourcesmay include a wide variety of numbers of light sources depending on thespecific requirements of the situation. Also, although the light sourceshave been described as LEDs, it should be understood that the presentinvention is not limited to LEDs but instead includes other forms oflight sources including, but not limited to, laser diodes, cold cathodeor field emitter cathodoluminescent sources and incandescent andflourescent lamps together with a switchable color filter such asDisplayteck's RGB Fast Filter color filter. Furthermore, each of thelight sources may be made up of a cluster of light sources such asseveral LEDs tiled together to form the light source. In a color versionof this embodiment, this cluster of light sources includes light sourcesof different colors tiled together to form each light source.

Still referring to FIG. 3, light sources 52 are spaced apart by aspecific distance D1 which produces real images of light sources 52 atsource imaging areas 54 that are spaced apart by a specific distance D2which can be easily calculated by those skilled in the optics art.Distance D1 is selected to be a distance which causes distance D2 to bea distance which is less than the diameter of a typical viewer's pupil,for example less than 3 mm, when the viewers pupil is adjusted to thebrightness of the display. This allows the viewer to view the virtualimage of the entire display so long as the pupil of the viewer's eye iswithin the overall source imaging area which includes all of sourceimaging areas 54 or within viewing region 56. For purposes of thepresent invention, this positioning of the images of the light sourcessuch that the viewer is able to view the virtual image of the entiredisplay so long as the pupil of the viewer's eye is within the overallsource imagine area is defined as substantially filling the sourceimaging area. By producing a plurality of images as shown in FIG. 3, theoverall source imaging area is enlarged. By controlling the distance D1that the light sources are spaced apart, the spacing of the images iscontrolled and therefore the overall size of the source imaging area iscontrolled. Also, the overall source imaging area may be furtherenlarged by increasing the number of light sources making up the arrayof light sources. This array of light sources enlarges the overallsource imaging area without increasing the size of the other opticscomponents or the size of the overall display system. Therefore, thedisplay system may, be scaled down in size without creating the problemof producing an exit pupil that is to small or, in other words, a sourceimaging area that has an area to small to be practically viewed asdescribed above.

In a specific example comparing the system shown in FIG. 3 to the basicsystem shown in FIG. 2B, the light source images at the source imagingareas are magnified by the ratio of the eyepiece focal length to thecollimating lens focal length when the light source is placed one focallength of the collimating lens from the collimating lens. With both thecollimating lens and the eyepiece lens having approximately the samediameter, about equal to the display diagonal, and using conventionallens technology, the magnification factor would typically be difficultto make much larger than a factor of two while maintaining a focallength for the eyepiece that provides the desired eye relief. Using anLED 0.25 mm square as the light source and a magnification factor of 2,the corresponding image would be 0.5 mm square. Therefore, the system,shown in FIG. 2B would form an image at source imaging area 0.5 mmsquare. With a source imaging area this small and using a viewer's pupildiameter of 3 mm, for example, it is clear that the viewer's pupil wouldmove out of the source imaging area during normal movement of the eye.However, using the arrangement designed in accordance with the presentinvention and shown in FIG. 3, a display of the same size using the samelenses and having each of the nine LEDs of the array spaced 1 mm apart,produces a source imaging area 4.5 mm square. This area includes thearray of nine 0.5 mm square images spaced 2 mm apart. Also, using thesame pupil diameter of 3 mm, the viewer's pupil would always be able toview at least one of the images as long as the pupil was positionedsomewhere within the source imaging area. As mentioned above, thissource imaging area would be further enlarged by increasing the numberof light sources making up the array.

Referring to FIG. 4 which illustrates a miniaturized display systemgenerally indicated by reference numeral 58, a second embodiment of thepresent invention will be described in detail. As shown in FIG. 4,miniaturized display 58 includes spatial light modulator 46, polarizingbeam splitting cube 48, eyepiece lens 36, and the array of individuallight sources 52 as were described above for FIG. 3. However, it,accordance with the present invention, display 58 includes an array or aplurality of individual collimating lenses which are indicated byreference numeral 60, each of which is associated with one of the lightsources 52 and each of which has a focal length much shorter than wouldbe possible using a single collimating lens as described above. In thisparticular embodiment, the array of collimating lenses includes threerows of three lenses. Each light source 52 is positioned one focallength of its associated collimating lens from its associatedcollimating lens. Each of these light sources 52 and their associatedcollimating lens 60, in cooperation with the other optics components,illuminate an associated portion of spatial light modulator 46 andproduce a portion of an overall virtual image of the spatial lightmodulator illuminated by the associated light source. Therefore, anoverall virtual image is formed which corresponds to overall spatiallight modulator 46. Although only three rows of three light sources andtheir associated collimating lenses are described, it should beunderstood that the array of light sources and their collimating lensesmay include a wide variety of numbers of light sources, which may be ofdifferent colors, and collimating lenses depending on the specificrequirements of the situation. Furthermore, each of the light sourcesassociated with each collimating lens may be made up of a cluster oflight sources such as several LEDs tiled together to form the lightsource. In a color version of this embodiment, this cluster of lightsources includes light sources of different colors all associated withone collimating lens.

By using the arrangement illustrated in FIG. 4 and as will be describedin more detail immediately hereinafter, two advantages are provided.First, using a plurality of collimating lenses allows for a shorteroptical path in the illuminator portion of the system reducing therequired size and bulk of this portion of the system. Second, by usingsmaller diameter collimating lenses, with corresponding shorter focallengths, the real image of sources 52 formed at a source imaging area 62is magnified by a factor proportional to the ratio of the focal lengthof the eyepiece lens relative to the focal length of the collimatinglens, which in this arrangement would be a significant magnification.

Referring to FIGS. 5A and 5B, a specific example of the above mentionedtwo advantages provided by the arrangement shown in FIG. 4 will bedescribed. FIG. 5A illustrates the unfolded optical path of the light ofthe arrangement shown in FIG. 2B while FIG. 5B illustrates the unfoldedoptical path of the light for a single light source in the arrangementdesigned in accordance with the present invention and shown in FIG. 4.Using the same lens focal length ratios as were used in the previousexamples, the arrangement shown in FIG. 5A results in a magnificationfactor of two. This is obtained by using eyepiece lens 22 having a focallength of 25 mm, the desired eye relief distance, and fasts collimatinglens 34 with a 12.5 mm focal length. Using the same 0.25 mm square LEDlight source 28, the resulting magnified image at source imaging area 38is 0.5 mm square as mentioned in the earlier example. However, as shownin FIG. 5B, because the diameter of the plurality of collimating lenses60 in overall display 58 are much smaller, a much smaller focal lengthmay be used. In this example, if the focal length of each of thecollimating lenses is reduced by a factor of four to 3.125 mm, (keepingthe focal length of the eyepiece at 25 nun this results in amagnification factor of 8 and an image at the source imaging area 62 of2 mm. As mentioned above, because the focal length of collimating lenses60 are reduced, light sources 52 may be moved in closer to the lenses,reducing the optical path length and the bulk of the illuminator portionof the overall display system. Furthermore, as mentioned above, itshould be understood that the array of light sources and collimatinglenses may have a wide variety of numbers of light sources andcollimating lenses. As the number of the light sources and associatedcollimating lenses is increased, both of the above described advantagesare further improved.

Referring to FIG. 6, a variation of the embodiment illustrated in FIG. 4will be described. In this variation, all of the components making upoverall display 58 are the same with the only difference being thepositioning of light sources 52 relative to collimating lenses 60. Asshown in FIG. 6, light sources 52 are positioned slightly closer tocollimating lenses 60 which causes the collimating lenses to directlight into spatial light modulator 46 in a slightly diverging manner.This results in several advantages in the overall display. First, sincethis causes the source imaging area to move further from eyepiece lens22, this increases the eye relief slightly, providing a more comfortableviewing position. Second, since the magnification factor is determinedby the ratio of how far the source imaging area is from the eyepiecelens which is increased in this case and how far the light source ispositioned from the collimating lens which is reduced in this case, themagnification is increased. This further enlarges the real image of thesource at the source imaging area. Third, since the light sources aremoved even closer to the collimating lenses the size of the illuminatorportion of the system is reduced still further as compared to the systemof FIG. 4. And finally, the slightly diverging light from each lightsource creates overlaps of the light from each light source on spatiallight modulator 46. This overlap improves the overall display byreducing dim spots in the virtual image of the display as well asreducing longitudinal vignetting, or in other words, reducing theproblem of losing view of the display if the viewer's pupil is movedfurther away from the display than the designed eye relief distance. Asan actual example, where the focal length of each collimating lens is3.125 mm, to accomplish the desired divergence, the cooperating lightsource could be positioned 3 mm or less from its collimating lens.

The repositioning of the light source as described above can only bedone to a limited extent. As light sources 52 are moved closer andcloser to collimating lenses 60 (which now no longer actually collinatethe light), the light is directed into polarizing beam, splitting cube48 in more and more of a diverging manner. Since polarizing beamsplitting cubes work most efficiently on light entering the cube at aspecific angle (in this case collimated light from the light sourceentering normal to the cube surface) the polarizing beam splitting cubedirects, or leaks, more and more light of the wrong polarization intothe spatial light modulator thereby reducing the contrast of thedisplay. Because of this limitation, light source 52 can only be moved alimited distance closer to collimating lens 60 without adverselyeffecting the contrast of the display.

Referring to FIGS. 7A and 7B, two specific light source arrangementsdesigned in accordance with the present invention will be described indetail. FIG. 7A illustrates a light source arrangement generallydestinated by reference numeral 64 which includes a glass substrate 66.An array of light sources 68, such as LED die, are attached to glasssubstrate 66. In the particular embodiment shown, three rows of threeLED die are attached to the glass substrate. An array of lenslets 70,each of which corresponds to an associated light source 68, are attachedto glass substrate 66 directly above their associated light sources.Arrangement 64 also includes an array of electrically conductive leads72 printed or otherwise attached to glass substrate 66 and adapted forconnection with a suitable power supply to provide electrical power toeach of light sources 68. In this arrangement, leads 72 may be providedas transparent leads made from, for example, indium-tin oxide. Althoughlight source arrangement 64 is described as having only three rows ofthree light sources, it should be understood that the array of lightsources may include a wide variety of numbers of light sources dependingon the specific requirements of the situation. Also, although the lightsources have been described as LEDs, it should be understood that thepresent invention is not limited to LEDs but instead includes otherforms of light sources including but not limited to laser diodes, coldcathode or field emitter cathodoluminescent sources and incondescent andflourescent lamps together with a switchable color filter such asDisplayteck's RGB Fast Filter color filter. Furthermore, each of thelight sources may be made up of a cluster of light sources such asseveral LED die tiled together to form the light source. In a colorversion of this embodiment, this cluster of light sources includes lightsources of different colors tiled together to form each light source.The focal length and positional arrangement between light sources 52 andlenses 60 described with respect to FIG. 6 may be maintained inarrangement 64 and arrangement 74 to be described immediately below.

Referring to FIG. 7B, an alternative embodiment of a light sourcearrangement designed in accordance with the present invention andgenerally designated by reference numeral 74 will be described. Lightsource arrangement 74 includes a substrate 76 having an LED wafer 7Sattached to one surface. This LED wafer 78 is a relatively large portionof an LED wafer which is not cut into small individual die as istypically done in the manufacture of LEDs, but instead, is a continuoussheet of LED wafer material, in this particular embodiment approximately25 mm square. A grid of electrically conductive leads 80 are formed onthe surface of LED wafer 78. Leads 80 may be either transparent oropaque depending on the requirements of the application and are adaptedto distribute electrical power from a suitable power supply over theentire surface of the wafer, substantially uniformly, such that whenpower is applied to the grid of leads, the entire LED wafer emits lightof substantially uniform brightness. Leads 80 may b applied to wafer 78using conventional screen printing or integrated circuit manufacturingtechniques. Although light source arrangement 74 has been described asbeing 25 mm square, it should be understood that this arrangement may beused to provide continuous light sources of a wide variety of sizes. Infact, a plurality of light source arrangements using LED wafers asdescribed immediately above may be tiled together to form very largelight sources depending on the requirements of the situation.

Still referring to FIG. 7B, if collimated light is desired for theapplication in which light source 74 is to be used, the grid ofelectrically conductive leads 80 may be formed using an opaque material.This opaque grid of leads effectively divides the wafer into an array ofindividual LED wafer portions or individual LED light sources, one ofwhich is indicated by reference numeral 82, with all of the LED waferportions are immediately adjacent one another. For this embodiment,light source arrangement 74 further includes an array of collimatinglenslets 84 overlaying the array of individual LED wafer portions 82 andformed within a single sheet 86. Each lenslet 84 is associated with acorresponding LED wafer portion 82 and is aligned with and positioneddirectly above its associated wafer portion. This arrangement provides anearly continuous sheet of LED light sources which emit collimated lightthrough their associated lenslets. Also, because this arrangement isvery thin, it is an excellent light source for use in a miniaturizedimage generating system. In fact, using a light source arrangement suchas arrangement 74 in an image generating system designed in accordancewith the present invention essentially eliminates the additional bulk ofthe overall system due to positioning the light source arrangement toone side of the overall system as described above.

Although the light source arrangement described above has been describedas being used in a miniaturized image generating system, it should beunderstood that this arrangement of and method for producing an LEDwafer light source in a relatively large sheet is not limited to thisspecific application. Instead, the LED wafer light source of the presentinvention may be used in a wide variety of applications which require athin, bright, evenly distributed light source.

Referring now to FIG. 8, another embodiment of an assembly for producingmodulated light designed in accordance with the present invention andgenerally designated by reference numeral 88 will be described. Assembly88 includes all of the components included in system 44 illustrated inFIG. 2B, that is, assembly 88 includes light source 28, spatial lightmodulator 46, source imaging area 38, collimating lens 34, polarizingbeam splitting cube 48, and eyepiece lens 36. However, in accordancewith the present invention, assembly 88 further includes an auxiliarypolarizer 90 positioned optically between collimating lens 34 antipolarizing beam splitting cube 48. Polarizer 90 improves the efficiencyat which the system directs light of only one polarization (in thiscase, for example, S-polarized light) into spatial light modulator 46.

Readily available polarizing beam splitting cubes, such as cube 48, arenot 100% efficient at directing only light of one polarization (forexample, S-polarized light) into spatial light modulator 46, in otherwords, cube 48 leaks some of the light of the opposite polarization (inthis case P-polarized light) into the modulator. This is especially trueif the light is not very well collimated and if the light includes avariety of wavelengths. The more collimated the light enteringpolarizing beam splitting cube 48 and the narrower the wavelength bandof light entering polarizing beam splitting cube 48, the more effectiveit is at directing only light of one polarization (S-polarized light)into the spatial light modulator. By adding auxiliary polarizer 90, thevast majority of light allowed to enter polarizing beans splitting cube48 is already of the one (S-polarized light) which is desired to bedirected into spatial light modulator 46. Therefore the amount of lightof the opposite polarization (P-polarized light) available to leak intospatial light modulator 46 is substantially reduced, increasing theoverall efficiency at which assembly 88 directs only light of onepolarization (S-polarized light) into spatial light modulator 46. Thisuse of an auxiliary polarizer improves the contrast of the imagegenerated by the overall image generating system and is equallyapplicable where multiple light sources are used.

The system illustrated in FIG. 9 illustrates the assembly for producingmodulated light shown in FIG. 8 which, in accordance with the presentinvention, further includes an auxiliary analyzer 92. Auxiliary analyzer92 is positioned between polarizing beam splitting cube 48 and eyepiecelens 36 and further improves the contrast of the system by blocking anylight of the one polarization (S-polarized light) which is intended tohave been reflected away from eyepiece lens 36 by polarizing beamsplinting cube 48 but leaked through the polarizing beam splitting cubebecause the cube is not 100% effective as described above. Usingauxiliary polarizer 90 and auxiliary analyzer 92 provides good contrastin the overall image generated by the system while relaxing therequirements on polarizing beam splitting cube 48 such that aconventional and readily providable polarizing beam splitting cube maybe used even if the light directed into the cube is directed into thecube in a slightly diverging manner and is made up of a variety ofdifferent wavelengths. If fact, using auxiliary polarizer 90 andauxiliary analyzer 92 allows a non polarizing beam splitter to be usedin place of polarizing beam splitting cube 48, although this is not aseffective as the system described above.

Referring to FIGS. 10 and 11, another embodiment of a miniature displaysystem generally designated by reference numeral 94 will be described.In accordance with the present invention, miniature display system 94includes light source 28, spatial light modulator 46, source imagingarea 38 and eyepiece lens 36 as have been described above for severalother embodiments. However, in this embodiment, light source 28 ispositioned adjacent to one of the edges of spatial light modulator 46which dramatically reduces the size of the overall system by essentiallyeliminating the illuminator portion of the optical path that in theprevious embodiments has been located off to one side of the axis normalto the spatial light modulator and eyepiece lens. Also, collimating lens34 and polarizing beam splitting cube 48 of FIG. 2B are replaced by (i)a suitable and readily providable curved surface beam splitter 96positioned between spatial light modulator 46 and eyepiece lens 36, (ii)an auxiliary, polarizer 98 positioned between light source 28 and curvedsurface beam splitter 96, and (iii) an auxiliary analyzer 100 positionedbetween curved surface beam splitter 96 and eyepiece lens 36. Curvedsurface beam splitter 96 is designed to reflect and collimate a portionof the light (in this case S-polarized light) from light source 28 afterit has passed through auxiliary polarizer 98 directing this light intospatial light modulator 46. Curved surface beam splitter 96 also isdesigned to transmit a portion of the light directed from spatial lightmodulator 46 to eyepiece lens 36 (in this case both S-polarized lightand P-polarized light). However, auxiliary analyzer 100 blocks lightwhich has not been converted to the opposite polarization (in this caseblocking S-polarized light) so that only light converted to the oppositepolarization (P-polarized light) by spatial light modulator 46 isallowed to pass into eyepiece lens 36.

Alternatively, as illustrated in FIG. 11, curved surface beam splitter96 is replaced with a curved surface polarizing beam splitter 102 whichincludes a surface coating which makes it a polarizing beam splitter.This eliminates the need for auxiliary polarizer 98 or auxiliaryanalyzer 100 or both polarizer 98 and analyzer 100. Both of thearrangements shown in FIGS. 10 and 11, in accordance with the presentinvention and as mentioned above, significantly reduce the bulk andweight of miniaturized display system 94. Also, since it is known in theprior art how to produce a curved surface beam splitter which would besuitable for these applications, all of the above described componentsare readily providable.

Turning now to FIG. 12, another variation of the immediately abovedescribed miniaturized display system generally designated by referencenumeral 104 will be described. Miniature display system 104 is identicalto system 94 shown in FIG. 11 except that curved surface polarizing beamsplitter 102 is replaced with a flat holographic polarizing beamsplitter 106 which serves the same purpose. Holographic polarizing beamsplitter 106 includes a diffraction grating which serves as the hologramwhich in turn serves as a beam splitter, a polarizer/analyzer, and as acollimator. It is known in the prior art how to produce a holographicpolarizing beam slitter which would be suitable for these applications,and therefore as mentioned above for FIG. 11, all of the componentsrequired for display system 104 are readily providable. One example ofsuch holographic diffusers are Physical Optics Corporation's LightShaping Diffusers™. As described above for other embodiments of thepresent invention, auxiliary polarizer 98 and auxiliary analyzer 100 maybe added to system 104. This would allow a holographic beam splitterwhich is not polarizing to be used in place of holographic polarizingbeam splitter 106 if desired.

In another variation of the immediately above described embodiment, FIG.13 illustrates a miniature display system designed in accordance withthe present invention and generally designated by reference numeral 108.In system 108, holographic polarizing beam splitter 106 of FIG. 12 isreplaced by an edge illuminated holographic optical element 110 andlight source 28 is replaced with at least one laser diode 112 positionedat the edge of holographic optical element 110. In this arrangement,holographic optical element 110 is a flat element with a relativelysmall thickness and is positioned adjacent to the top surface of spatiallight modulator 46 so that it covers the entire light modulatingsurface. Laser diode 112 directs light into at least one edge ofholographic optical element 110 which is constructed with a refractiveindex grating. This refractive index refracts the light in a controlledway to evenly illuminate spatial light modulator 46. In one variation ofthis embodiment, holographic optical element 110 also acts as thepolarizer and analyzer by directing only light of one polarization intospatial light modulator 46 and only allowing light of the oppositepolarization to be transmitted through it from spatial light modulator46. Alternatively, as described above for other embodiments, auxiliarypolarizer 98 and auxiliary analyzer 100 may be added eliminating theneed for holographic optical element 110 to act as the polarizer andanalyzer.

As shown in FIG. 13, the size of miniature display system 108 is able tobe reduced even further than any of the above described arrangements.First, because the laser diodes are positioned immediately adjacent toholographic optical element 110 the length of the optical path betweenthese elements is minimized. Second, since holographic optical element110 provides all the functions of polarizing beam splitting cube 48 ofFIG. 2B, and because holographic optical element 110 is so thin, theoptical path between spatial light modulator 46 and eyepiece lens 36 isalso minimized.

Although in each of the above described embodiments illustrated in FIGS.10-13, the light source has been illustrated as being a single lightsource, it should be understood that the light source may include aplurality of light sources. In fact, as described above for otherembodiments, in color versions of these embodiments, the light sourcewould include light sources of different colors. For example, in FIG.13, light source 12 may include a plurality, of laser diodes ofdifferent colors.

Referring now to FIGS. 14A and 14B, another embodiment of a miniaturedisplay system designed in accordance with the present invention andgenerally designated by reference numeral 114 will be described. Asshown in FIG. 14A, display 114 includes at least one light source 116, apolarizer 118, spatial light modulator 46 having a light receivingplanar surface 120, an eyepiece lens 122, and an analyzer 124. Inaccordance with the present invention, light source 116 is positionedadjacent to the perimeter of eyepiece lens 122 and directs light throughpolarizer 118 such that light of one polarization (in this caseS-polarized light) is directed into spatial light modulator 46 at anacute angle to an axis 126 normal to light receiving surface. Spatiallight modulator 46 modulates the light converting certain potions of thelight to the opposite polarization (P-polarized light) and directs thelight into eyepiece lens 122. Eyepiece lens 122 is positioned off axisfrom axis 126 normal to the center of spatial light modulator 46 andtherefore is a suitable and readily available asymmetrical lens.Eyepiece lens 122 directs the light from spatial light modulator 46 tosource imaging area 128 through analyzer 124. Analyzer 124 blocks lightwhich has not been converted to the opposite polarization (blocksS-polarized light) so that only the converted light is directed tosource imaging area 128.

As shown in FIG. 14B, light source 116 may include a plurality ofindividual light sources positioned at discrete locations around theperimeter of the eyepiece lens. In this arrangement, a symmetrical lens,such as eyepiece lens 36, is positioned on axis with axis 126 normal tothe center of spatial light modulator 46. As mentioned above for otherembodiments, light source 116 used in both FIGS. 14A and 14B may beprovided in a variety of specific forms such as, but not limited to, anLED, a laser diode, or a variety of other such devices. Furthermore,each of the light sources may be made up of a cluster of light sourcessuch as several LEDs tiled together to form the light source. In a colorversion of this embodiment, this cluster of light sources includes lightsources of different colors tiled together to form each light source.

Turning to FIGS. 15A and 15B, a variation of the miniature displaysystem described immediately above will be described. As shown in FIG.15A, miniature display system 130 includes all of the componentsdescribed above for the miniature display system illustrated in FIG. 14Aexcept that symmetrical eyepiece lens 36 is used instead of asymmetricallens 122. However, in system 130 of FIG. 15A light source 116 andpolarizer 118 are positioned between spatial light modulator 46 andeyepiece lens 36. This arrangement causes the problem that the viewer'sview of the spatial light modulator is partially blocked by light source116. However, if spatial light modulator 46 uses a weakly diffusedmirror rather than a specular mirror, this problem is minimized.Alternatively, as shown in FIG. 15B, this problem is overcome by using aplurality of light sources 116 and cooperating polarizers positionedbetween spatial light modulator 46 and eyepiece lens 36. This pluralityof light sources may be provided by an arrangement such as light sourcearrangement 64 described above and illustrated in FIG. 7A.

Referring to FIGS. 16 and 17, two variations of an assembly forproducing modulated light designed in accordance with the presentinvention and generally designated by reference numerals 132 and 134will be described. As shown in FIG. 16, assembly 132 includes lightsource 28, spatial light modulator 46, collimating lens 34, polarizingbeam splitting cube 48, and eyepiece lens 36 as described above for FIG.2B. However, assembly 132 further includes a mirror 136 positioned onthe opposite side of cube 48 relative to light source 28. Mirror 136 ispositioned to reflect the light of the polarization which is transmittedthrough polarizing beam splitting cube 48, in this case P-polarizedlight, back to a point 138 immediately adjacent but to one side of lightsource 28. Assembly 132 also includes a quarter wave plate 140 and amirror 142 positioned at point 138 which convert the light of thepolarization which is transmitted by cube 48 (P-polarized light) tolight of the opposite polarization (S-polarized light) and redirects thelight back into cube 48. This arrangement doubles the amount of lightused from light source 28 by not wasting the half of the light which isof the polarization that is transmitted by cube 48. Also, since mirror136 reflects the light back to point 138 immediately adjacent to lightsource 28, this arrangement in effect provides a second light sourcewhich provides the benefits described above for the arrangementillustrated in FIG. 3 where multiple light sources are provided.Alternatively, if multiple light sources are used in this arrangement,it effectively doubles the number of light sources, again providing theabove described advantages.

FIG. 17 illustrates an alternative assembly 134 to avoiding the wastingof light from light source 28. As shown in FIG. 17, polarizing beamsplitting, cube 48 is replaced by a first and a second smallerpolarizing beam splitting cubes, indicated by reference numerals 144 and146 respectively, each of which is positioned over a correspondingportion of spatial light modulator 46. Also, mirror 136, mirror 142, andquarter wave plate 140 are replaced bad half wave plate 148 positionedbetween the two polarizing beam splitting cubes 144 and 146. In thisarrangement, light from light source 28 is directed into firstpolarizing beam splitter 144 by collimating lens 34. Cube 144 directslight of one polarization, in this case S-polarized light, down into itsassociated portion of spatial light modulator 46 and allows light of theopposite polarization (P-polarized light) to pass through cube 144.Since half wave plate 148 is positioned between cube 144 and cube 146,the light which is transmitted through cube 144 (P-polarized light) isalso transmitted through half wave plate 148 which converts thepolarization of the light passing through it to the oppositepolarization (S-polarized light). Therefore, the light entering cube 146is essentially all light of the polarization (S-polarized light) whichcube 146 directs down into its associated portion of spatial lightmodulator 46. This arrangement provides the benefit of not wasting lightof ore polarization from light source 12 and also significantly reducesthe bulk of the overall assembly by reducing the bulk of the polarizingbeam splitting cubes.

Referring to FIGS. 18A-C, another presently preferred embodiment of aminiaturized display system designed in accordance with the presentinvention and generally designated by reference numeral 150 will bedescribed. As shown in FIG. 18A, display 150 includes spatial lightmodulator 46, polarizing beam splitting cube 48, and eyepiece lens 36 ashave been described above for several other embodiments. However,display 150 further includes a light source 152 surrounded by areflector 154; a diffusing plate 156 positioned between light source 152and polarizing beam splitting cube 48; a Fresnel collimating lens 158positioned between diffusing plate 156 and cube 48, a black plastichousing 160 surrounding and supporting light source 152, diffusing plate156, and Fresnel lens 158; and a source imaging area 162. Fresnel lens158 is used in this embodiment because it is less expensive, lighterweight, and is able to be constructed with a shorter focal length than aconventional lens of the same diameter.

In a monochrome version of this embodiment, diffusing plate 156 diffusesthe light from light source 152, which is made up of a plurality oflight sources. As shown best in FIG. 18C, plastic housing 160 supportsdiffusing plate 156 at a specific distance L away from light source 152between light source 152 and Fresnel lens 158. Also as shown in FIG.18C, the light emitting portions of the LEDs are spaced apart a certaindistance d and emit light at a certain angle A. As will be described inmore detail immediately hereinafter, this arrangement provides the bestresults when diffusing plate 156 is a weak diffuser and is placed atleast a distance L from the light source. This distance L is determinedby the equation L≧d/A. This arrangement provides the proper mixing ofthe light source 152 so that the light from light source 152 provides asubstantially uniform brightness of light throughout source imaging area162.

In a color version of this embodiment, light source 152 is made up of aplurality of different colored LEDs, in this case, three green LEDs 164,two red LEDs 166, and two blue LEDs 168, all positioned immediatelyadjacent to one another. Reflector 154 surrounds all seven LEDs andhelps direct the light from the LEDs toward Fresnel collimating lens156. In the color version, diffusing plate 156 is positioned a distanceL from light source 152 such that there is sufficient mixing of thelight from the different color light sources so as to be able to achievea substantially uniform white light throughout source imaging area 162.

When operating the color version of the miniaturized display systemshown in FIG. 18A, light of each of the three colors is directed intospatial light modulator 46 of different times and modulated to producethe proper gray scale image desired for that particular color. The threecolors are cycled at a frame rate or speed sufficiently fast to causethe viewer's eye to integrate the three different colored gray scaleimages into an integrated color image. Because the exact location ofeach of the three different colored LEDs making up light source 152 arespaced apart by distance d, if diffusing plate 156 were not included,each LED would form a corresponding image at source imaging area 162which is spaced apart from the images formed by the other LEDs asdescribed above for FIG. 3. This is not a problem for a monochromaticdisplay since all of the images would be the same, however, with a colordisplay this would result in shifts in the color of the perceived imagewith movement of the viewer's pupil. This problem is solved by placingdiffusing plate 156 between light source 152 and Fresnel lens 158 asmentioned above.

The specific positioning and the diffusing strength of diffusing plate156 have a significant impact on the performance of the system. Asmentioned above, the best results occur when a weak diffuser ispositioned at least a distance L away from light source 152. Thispositional relationship between the distance from the light source atwhich the diffuser is placed, the distance between the individual lightsources, and the angle at which the light sources emit light causesenough overlap of each of the light sources at the diffusing plate suchthat when the light is weakly diffused, the images formed at sourceimaging area 162 are properly mixed minimizing the color registrationproblem described above.

Although the above described display system has been described includinga single light source made up of seven LEDs adjacent to one another, itshould be understood that the present invention is not limited to onesuch light source. Instead, the light source may be made up of aplurality of light sources as described for FIG. 3 with each lightsource including light sources of different colors. Also, although inthe above described example seven LEDs were used, the present inventionwould apply regardless of the specific number of LEDs used andregardless of the specific type of light source used. For example, theLEDs may be replaced with laser diodes, cold cathode or field emittercathodoluminescent sources, incondescent and flourescent lamps togetherwith a switchable color filter such as Displayteck's RGB Fast Filtercolor filter, or any other appropriate light source. Furthermore,although the above described display has been described as including asingle collimating lens, it should be understood that, as described forFIG. 4, this embodiment may incorporate a plurality of collimatinglenses. In fact, the light source used in this embodiment may beprovided by a light source as described above for FIG. 7A in which aplurality of light sources, such as LEDs, are attached to a substrate toform an overall light source.

Referring to FIG. 19, another arrangement for improving the performanceof a color version of an image generating system will be described. FIG.19 illustrates a portion of a miniaturized image generating systemincluding light source 170. As described above for FIG. 18A-C lightsource 170 includes green light source 164, red light source 166, andblue light source 168. As has been described above for severalembodiments, this system includes a collimating lens 34 and a polarizingbeam splitting cube 48. As mentioned above, polarizing beams splittingcubes are not 100% efficient, and their efficiency is dependent on theangle at which the light enters the cube and the wavelength of thelight. As will be described immediately hereinafter and in accordancewith the present invention, light sources 164, 166 and 168 can bestrategically positioned to improve the performance of polarizing beamsplitting cube 48.

As shown in FIG. 19, since light sources 164, 166, and 168 can not allbe positioned at the focal point of collimating lens 34 and are slightlyspaced apart, the light emitted from each light source is directed intopolarizing beam splitting cube 48 at slightly different angles. In thisexample, green light source 164 is positioned at the focal point of lens34 which collimates the green light, indicated by lines 172, and directsthe light into cube 48 perpendicular to the cube. Also, in this example,polarizing beam splitting cube 48 is tuned to the wavelength of thegreen light emitted by source 164. That is, a polarizing beam splittingfilm 174 positioned diagonally within cube 48 is designed to have acertain thickness t that works most efficiently when light of thewavelength of source 164 is directed into cube 48 perpendicular to cube48 as shown in FIG. 19.

Still referring to FIG. 19, red light source 166 is positioned abovegreen light source 164 a certain distance D3. Red light emitted fromlight source 166 is collimated by lens 34 and directed into cube 48 at aparticular angle A1 which is dependent on distance D3 as indicated bylines 176. Because polarizing beam splitting film 174 is positioneddiagonally within cube 48 and because red light 176 is directed intocube 48 at angle A1, red light 176 must pass through a larger distanceof film 174 than green light 172 since red light 176 intersects film 174at a larger incident angle than green light 172. Therefore, since redlight has a longer wavelength than green light, distance D3 may beselected to optimize angle A1 and cause red light 176 to intersect film174 at an angle that improves the efficiency at which film 174 acts onred light 176. This same general approach may be used for blue lightsource 168 positioned a distance D4 below green light source 164. Thiscauses blue light emitted from light source 168 to be collimated by lens34 and directed into cube 48 at angle A2 as indicated by lines 178. Bluelight 178 intersects film 174 at a smaller incident angle than greenlight 172 which results in blue light 178 passing through a smallerdistance of film, 174 than green light 172. Since blue light has ashorter wavelength than green light, distance D4 may be controlled toimprove the efficiency at which cube 48 acts on blue light 178.

Although the above example has been described using red, green, and bluelight, it should be understood that the present invention is not limitedto these specific colors. Also, although only three colors weredescribed, the present invention would equally apply regardless of thenumber of colors of light being used. Furthermore, this general approachof strategically placing light sources of different colors to improvethe efficiency of a polarizing beam splitting cube would equally applyto other embodiments which replace the polarizing beam splitting cubewith other elements. For example, this general approach has particularsignificance for the embodiment of the present invention shown if FIG.12 where the polarizing beam splitting cube is replaced with an edgeilluminated holographic optical element.

Although only several specific embodiments of the present invention havebeen described in detail it should be understood that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. For instance, each of theinventive features of the various embodiments described may be combinedin a wide variety of ways. As mentioned above, although most of theembodiments described used LEDs as the light source, it should beunderstood that a variety of types of light sources may be used in placeof the LEDs such as laser diodes, cold cathode or field emittercathodoluminescent sources, incondescent and flourescent lamps togetherwith a switchable color filter such as Displayteck's RGB Fast Filtercolor filter, and a variety of other light sources. Also as mentionedabove, although many of the embodiments were described as includingindividual light sources, such as LEDs, it should be understood thethese light sources may be made up of a cluster of light sources tiledtogether to form the light source and the cluster of light sources mayinclude light sources which emit light of different colors therebyproviding a color version of the system. Furthermore, although apolarizing bean splitting cube has been used in several examples, thisis not necessarily a requirement of the present invention. Other beamsplitters may be used in combination with an auxiliary polarizer and anauxiliary analyzer. However, applicants have found that when using aspatial light modulator which modulates light by changing thepolarization of the light, a polarizing beam splitter is more efficientthan other beam splitters even when auxiliary polarizers and analyzersare used because the polarizing beam splitter only wastes light of onepolarization.

Therefore, the present examples are to be considered as illustrative andnot restrictive, and the invention is not to be limited to the detailsgiven herein, but may be modified within the scope of the appendedclaims.

What is claimed:
 1. A display system, comprising: an LED illuminatorthat provides light of at least three colors; and an array of pixelsthat are illuminated by the LED illuminator, the pixels being switchablebetween optical states, in approximately 2 milliseconds or less; whereinthe array of pixels and the LED illuminator are controlled tosequentially display first an image of one of the at least three colors,followed by an image of a second one of the at least three colors,followed by an image of a third one of the at least three colors, inorder to form a color gray-scale image.
 2. A display system as definedin claim 1, wherein the array of pixels are part of a microdisplay.
 3. Adisplay system as defined in claim 2, wherein the microdisplay is areflective microdisplay.
 4. A display system as defined in claim 2,wherein the microdisplay is a liquid-crystal microdisplay.
 5. A displaysystem as defined in claim 2, wherein the microdisplay is aferroelectric liquid-crystal microdisplay.
 6. A display system asdefined in claim 1, wherein the LED illuminator includes 3 LEDs.
 7. Adisplay system as defined in claim 6, wherein the LED illuminatorincludes a red LED, a green LED, and a blue LED.
 8. A display system asdefined in claim 1, further including an optical element to focus thelight from the pixels.
 9. A display system as defined in claim 1,further including an optical element to create a magnified image of thepixel array.
 10. A display system, comprising: an LED illuminator thatprovides light of at least three colors; an array of pixels that areilluminated by the LED illuminator, the pixels being switchable betweenoptical states, in approximately 2 milliseconds or less; and an opticalelement to create a magnified image of the pixel array; wherein thearray of pixels and the LED illuminator are controlled to sequentiallydisplay color gray-scale images; wherein the optical element includes aneyepiece lens.
 11. A display system as defined in claim 1, furtherincluding optical elements that create a real image of at least aportion of the LED illuminator from the light provided by theilluminator.
 12. A display system as defined in claim 11, wherein thereal image is created from light that has passed through or beenreflected by the array of pixels.
 13. A display system as defined inclaim 3, wherein the microdisplay is a liquid-crystal microdisplay. 14.A display system as defined in claim 13, wherein the microdisplay is aferroelectric liquid-crystal microdisplay.
 15. A display system asdefined in claim 3, wherein the array of pixels is embodied on asemiconductor substrate.
 16. A display system as defined in claim 1,wherein the array of pixels change their optical state withoutmechanical motion.
 17. A display system as defined in claim 1, whereinvoltages applied to the pixels in the array of pixels to control theoptical state thereof are 5 volts or less.
 18. A display system asdefined in claim 1, wherein the array of pixels includes rows of pixelsand wherein the rows have a pitch of 10 μm or less.
 19. A display systemas defined in claim 1, wherein at least a portion of the time that theoptical state of the pixels is changed, the change is made while theilluminator is illuminating the pixels.
 20. A display system as definedin claim 1, wherein the controlling is performed in such a way that atleast some of the pixels switch between optical states whileilluminated.
 21. A display system as defined in claim 1, wherein thepixels are switchable between electrical states and wherein the opticalstate of each pixel is determined solely by its present electricalstate.
 22. A display system as defined in claim 21, wherein each pixelis switchable between only two electrical states.
 23. A display system,comprising: an LED illuminator that provides light of at least threecolors; and an array of pixels that are illuminated by the LEDilluminator at least approximately 50 percent of the time that thedisplay system is operating; wherein the array of pixels and the LEDilluminator are controlled to sequentially display first an image of oneof the at least three colors, followed by an image of a second one ofthe at least three colors, followed by an image of a third one of the atleast three colors, in order to form a color gray-scale image.